The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 95-517 (35 USC 202) in which the Contractor has elected to retain title.
1. Field of the Invention
The present invention describes a polymeric gas sensor. More specifically the present invention describes a polymeric gas sensor utilizing a variety of electrode geometries to generate varied responses to selected gases.
2. Background and Summary of the Invention
Atmospheric monitoring is increasingly required for industrial processes, environmental control, and health and safety reasons. Improved gas sensors have opened up new applications such as: control of air quality in places such as vehicle interiors, high traffic areas, coal mines and spacecraft; monitoring and control of industrial processes; ah early detection of fires; flue gas surveillance; breath analysis for diagnostic and therapeutic purposes; and control of combustion processes for both economic and environmental benefits.
Conventional techniques for gas sensing included the use of mass spectrometers, which are too large for many applications. Solid state gas sensors are increasingly used because they offer a number of advantages including small size and relatively low cost. Solid state gas sensors generally fall into three broad categories. Catalytic gas detectors burn flammable gases and detect the concentration of the flammable gas by the rise in temperature. Solid electrolyte sensors use ionic conduction to allow the generation of a concentration cell electro motive-force (EMF) between conducting species in solid and gas phases. Semiconducting oxide sensors employ the semiconducting properties of materials either in the form of high surface area, porous bodies, or in the form of thin, dense films. These include Silicon Carbide (SiC) and tin dioxide (Sn02) sensors.
Solid state gas sensors usually need to operate at relatively high temperatures. For example, most semiconducting oxide sensors operate at a few hundred degrees Celsius. High operating temperatures usually require high power consumption. This limits the usefulness of the sensor, particularly for battery powered applications. Thus it is an object of the present invention to provide a low power, compact, gas sensor array which operates at room temperature.
Polymer-based (xe2x80x9cpolymericxe2x80x9d) gas sensors use a sensing material formed of thin films of conducting polymers. These sensors use organic polymers to detect gases via conductivity changes. Polymeric gas sensors (also referred to herein as xe2x80x9cchemoresistorsxe2x80x9d), offer significant advantages. They operate at near room temperatures, and hence can be used for low power applications. Also, polymer-based sensors can be made to be very compact.
Polymeric gas sensors can be built into an array of sensors, where each sensor is designed to respond differently to different gases. For example, the gas sensitivity of each polymer in such an array is usually determined by the material""s polymerization and dopant characteristics. Each element of the array measures a different property of the sensed sample. Each different gas sample presented to the sensor array hence produces unique patterns of collective sensor element responses. These patterns become xe2x80x9csignaturesxe2x80x9d that are characteristic of a gas, or combination of gases. Sensor arrays may be combined with an automated data analysis system (such as a neural network) to identify specific vapors from the sensor output signatures. This kind of a gas sensor is often referred to as an xe2x80x9cartificial nose.xe2x80x9d Because they are very sensitive in identifying different gases, artificial noses are opening up many new applications; these include monitoring food and beverage odors, automated flavor control, analyzing fuel mixtures, quantifying individual components of gas mixtures, etc.
It is important to perform real-time sensing and monitoring of hydrocarbon gases affecting the health of persons in isolated environments. For example, spacecraft often require sensing of hydrocarbon gases that are a byproduct of decomposition of hydrocarbon based chemicals. However, current low power sensors that detect hydrocarbons are not able to effectively distinguish among different hydrocarbon compounds. This suggests a need for low-power sensors that can effectively distinguish among different hydrocarbons.
The present inventors have recognized that polymer-based gas sensors have the potential to meet this need. However, the implementation of polymer-based gas sensor arrays is presently limited by a lack of reproducibility of their response to gases. The present inventors have recognized that the development of a practical, reproducible, polymer-based gas sensor array will require full characterization of the conductivity of the polymer films used in the array. This characterization will preferably include measurements of various aspects of the conduction mechanisms in the polymer film used in the sensors. These conduction parameters include sheet resistance, surface conduction, anisotropic conduction, film nonuniformity, and contact resistance. It is an object of the present invention to provide a technique for characterizing conduction parameters of polymer films used with polymeric gas sensor arrays and to determine the optiumum electrode geometry.
It is another object of the invention to use these fully characterized, reproducible polymer films to construct low power, hydrocarbon sensitive polymer-based gas sensor arrays.
The present inventors have discovered that the response of polymeric films to various gases depends on the shape of the sensor electrode in ways that enable reproducible characterization of the polymeric films. In particular, different geometric configurations of the electrodes in the polymeric films result in different characteristic responses to gases. These reproducible and distinctive responses facilitate the detection of gases because they produce unique and consistent patterns that are readily recognizable.
One aspect of the invention is a method of measuring responses of a gas sensor to predetermined gases. The method includes the steps of fabricating a gas sensor that has a plurality of resistor elements that share a common electrode. Each resistor element has a plurality of different electrode configurations. For example, in one embodiment these electrode configurations include: U-bend, comb and serpentine shapes. The resistor elements are then exposed to a predetermined known gases. Patterns of characteristic changes in the conductance of the resistor elements in response to exposure to the predetermined gases is then measured and stored.
Another aspect of the invention is a gas sensor fabricated on a substrate that includes a plurality of electrodes where each electrode has a plurality of shapes an dimensions. Polymer films are deposited over the electrodes. The polymer films have a conductivity that changes when the film is exposed to various gases as measured across the electrodes. A current measuring circuit is connected to the electrodes to measure these conductivity changes. An optimal geometric configuration for the electrode is selected based on prior tests of the film""s response to various gases of interest. By having a electrode geometry as well as other factors that optimizes the polymer film""s response to a particular gas, the gas sensor exhibits an improved ability to distinguish among different gases. The test sensor of the present invention permits the optimization of numerous other factors such as polymerization process by control of the polymerization heat using the heater elements, polymer film doping and other factors.
Another aspect of the present invention relates to the fabrication of gas sensors. Prior fabrication techniques utilizing gas polymer films. Previous gas sensors employed polymer films that needed to be patterned once applied. These included Bridge and Van Der Pauw resistor structures. The fabrication technique of the present invention does not require patterning after application of the polymer film. In this fabricating technique a plurality of electrodes having the desired geometric configurations are first deposited on the substrate. A polymer film is then deposited over the electrodes. A heater element may also be deposited prior to the polymer film to permit control of the polymerization process. The result is a simpler and less expensive fabrication process than those requiring patterning.